Our list has morphed over the years, but here goes:
small first aid kit with bandaids and antibacterial ointment
tiny bottle containing various OTC meds (tylenol, excedrin, advil)
roll of Tums, bubble pack package of Pepto Bismol tablets, and imodium
protective case for glasses on big rides
sunscreen
camera
cell phones and/or walkie talkies
ponchos
various size ziploc bags
extra socks
facecloth or small towel
misting fan filled with ice
small bottles of frozen water (the little ones designed to go in lunch boxes)
car-washing chamois (DH puts one in his hat and it keeps him dry all day!)
small package of Wet Ones and/or baby wipes
mini roll of paper towels (found in sample section at Wal Mart)
mini bottle of baby powder (also found in sample section)
I program all of our ADRs into my cellphone and text message the info for that day to the rest of the family in the morning. We keep our tickets and room keys in our lanyards (great for fastpasses, too!) and ALWAYS have good shoes to wear (I usally alternate between good sneakers and
crocs from day to day). We all apply sunscreen before going out and wear hats and sunglasses.
The chamois is the one thing DH swears by. I buy a few of the synthetic ones at Wal Mart and usually cut them down into several smaller ones. They are machine washable, so I just toss them in with the clothes at the end of the day. He'll line his hat with them (he shaves his head and hates sweat dripping into his eyes) and we'll have a few in the backpack for wet seats, spills, etc. They are terrific!
Good luck and have fun!